Add missing documentation for pivot distinct argument

[tripleaceme]

Summary

• Adds the missing distinct argument to the pivot macro's documentation in README.md
• The distinct parameter (default: false) controls whether DISTINCT is used inside the aggregation function

Closes #1054

Test plan

• Verified the distinct argument exists in the macro signature (macros/sql/pivot.sql) with default value False
• Confirmed the new docs entry follows the same format as existing args